The coloring of utility glass with metallic nanoparticles and/or metal oxides is known.
On account of their particle size, which lies typically in the range from 1 to 100 nm, the metallic nanoparticles that are used as colorants for glass coloration have advantageous chemical and physical properties relative to conventional metals, examples being very high absorption coefficients (plasmon absorption) and a broad absorption band in the visible region of the electromagnetic spectrum, and hence an intense color.
The metal oxides that are also used as colorants, in contrast, have significantly lower absorption coefficients.
In the conventional coloring of glass, metals and/or metal salts are mixed with the other raw materials used for glassmaking, prior to the melting procedure. Customary raw materials for producing glass are quartz sand (SiO2) as network former, sodium carbonate and potassium carbonate (Na2CO3, K2CO3) for lowering the melting point, and also feldspar, dolomite, lime, and recycled glass. In order to improve certain properties, red lead, borax or barium carbonate are sometimes added. Customary colorants for producing colored glass are oxides of iron (yellow to brown), copper (red to blue), chromium (green), uranium (yellow to green), cobalt (blue), nickel (reddish), manganese (brown), and selenium (red).
For glass coloration by means of metallic nanoparticles, metals or metal salts of the elements silver (yellow), copper (red), and gold (red) are added to the starting materials for glassmaking. The use of nanoparticles of gold is one of the oldest techniques for coloring glass, having already been used for many hundreds of years in order to produce what is called cranberry glass, for church windows, for example. For glassmaking in this context, the starting materials are mixed in any of a wide variety of proportions. Examples of commonplace glass compositions are so-called soda-lime glass (SiO2 72%, Al2O3 2%, Na2O 14%, CaO 10%), float glass (SiO2 72%, Al2O3 1.5%, Na2O 13.5%, CaO 8.5%, MgO 3.5%) or lead crystal glass (SiO2 60%, Al2O3 8%, Na2O 2%, K2O 12%, PbO 17.5%).
The raw mixture is supplied to the melting operation. For this purpose, the mixture is heated in a first step until the materials melt, and then the gases formed are driven out by a further increase in temperature. The refined glass melt is then passed on for shaping. In conventional glass coloration, metallic colorants, such as metallic nanoparticles, for example, can be accommodated by the glass to be colored only up to an amount of approximately 0.1 percent by weight, based on the total mass of the glass, as is described, for example, in the dissertation by Thomas Rainer, Halle University, 2002, and in the literature cited therein.
The glass melt comprising metallic colorants is cooled and colored by a subsequent temperature treatment. In the course of this temperature treatment, nanoparticles are formed in the glass, and color the glass. The addition of colorant to the initial mixture prior to melting is also referred to as primary doping. With this technique of glass coloration, the colorants are distributed uniformly, as a result of the production process, throughout the volume of the glass.
In another known glass coloration procedure, the colorant is not introduced until after the melting operation, in other words into the solidified glass, by means of diffusion processes. This conventional glass coloring technique is also referred to as secondary doping. For this purpose, the glass to be colored is coated with a paste comprising the colorant, metal salts for example. During a subsequent temperature treatment, the colorant diffuses from the paste into the glass. The paste is subsequently removed from the cooled glass, and, by virtue of a further heat treatment, the glass is colored by the nanometric metal particles that form in the glass.
Another option to secondary diffusion is to mix the glass to be colored with the metallic colorants and to color the glass, doped with metallic colorants that have diffused into the glass from the mixture, by means of a subsequent heat treatment. A method of this kind is known from WO 2008/125857, for example. With this method, a disadvantage is that very high temperatures have to be applied, resulting in partial softening and, consequently, in partial deformation of the glass particles to be colored. Particularly in the case of glass platelets which are used as a substrate for producing effect pigments, more particularly pearlescent pigments, these deformations of the substrate result in a deterioration in the optical properties of the pearlescent pigments that are produced from them. It is therefore not possible to produce high-quality pearlescent pigments on the basis of colored glass platelets produced in accordance with the method of WO 2008/12857.
Also known is the doping of glass with metallic colorants by means of high-energy radiation (ion implantation). Since, however, this method is very energy-intensive, it is not employed in industrial production. Furthermore, it is not possible with this method to produce colored glass particles.
Known, for example, from DE 198 41 547 A1 is the coloring of utility glass by means of laser irradiation. For glassmaking with permanently colored structures, the glass is doped with metal ions, such as gold, copper, and silver, for example, and subsequently, by means of laser radiation, which causes no damage to the glass itself, the metal ions are reduced by local supply of energy, with oxidation of substances present within the glass. Through agglomeration, the reduced metals then form nanoparticles, which color the glass at the local sites. The method, however, can be employed only with flat glass, not with glass particles.
Furthermore, DE 101 19 302 A1 discloses a method for the laser-assisted introduction of metal ions through ion exchange and diffusion for the colored interior marking of glass for identification and advertising purposes. For this method, the glass surface, contacted with a material comprising metal ions, is heated locally by means of focused laser radiation, causing the metal ions to diffuse into the glass and be reduced to atoms. Through subsequent aggregation of the metal atoms to form metal particles, the glass is colored at the sites containing the metal particles. A method of this kind, however, can again be used only with flat glass, i.e., not with glass particles. Furthermore, in the glass sheets colored in this way, the metal nanoparticles are situated only in the outermost boundary layer of the glass, i.e., there is no uniform glass coloration in the depth.
WO 2007/031151 A1 relates to the production of colored glass. For this purpose, metal ions present in the glass or introduced are reduced by means of locally confined introduction of energy, by means of focused laser radiation, and are converted into a locally confined collection of metal particles, producing a local discoloration of the glass that is typical of the metal employed. In order to produce multicolor structures in the glass, in a first step, a plurality of locally confined regions with a monochromic initial coloration of different degrees of the same color is generated, and in a second step, as a result of a variable introduction of energy in the regions, particle formation processes and/or particle growth processes are induced that are dependent on the initial coloration, thereby giving the glass regions with a different regional coloration a different color. This method as well is not suitable for the production of colored glass particles.
JP 59199553 A, furthermore, relates to a glass which is filled with metal oxides or metal particles and which is subsequently encased. The metal oxides are, for example, chromium oxide and copper oxide; the metal particles consist of silver and gold, among others. The metal oxides and metal particles are added to the starting material and are therefore present only at low concentrations.
US 2003/0099834 A1 discloses photochromic glass nanoparticles comprising nanoparticles of silver halides which become dark on irradiation and become light again after the end of irradiation. The particles are precipitated from a microemulsion by precipitation. A disadvantage of the method is that, rather than coloredness, only a light/dark effect is generated. Furthermore, the microemulsion method is not suitable for the production of colored glass particles.
DE 44 11 104 A1 relates to a method for producing purple pigments on the basis of colloidal gold on firable support materials, in which, inter alia, an aqueous solution or suspension of a gold compound and a support material, consisting in particular of glass fluxes, are brought into contact with one another, and the mixture is subsequently treated thermally at a temperature above the decomposition temperature of the gold compound and below the sintering temperature of the support material, the gold compound being converted into colloidal gold. Disadvantages of the method, however, are that the gold is located merely on the surface of the substrate and not in its interior, making subsequent coatings very difficult, and that the gold is not protected by the glass from mechanical and/or chemical exposure.
EP 1 510 506 B1 discloses a glass flake with a specific glass composition and a low sodium fraction. A glass coating of gold is stated by way of example. A disadvantage, again, is that the gold is located on the surface and not in the interior of the substrate.
EP 1 837 379 A2 discloses a method for coloring glass platelets for use as fillers in cosmetics. For this purpose, inorganic colorants, especially cations or complex anions of the elements copper, chromium, manganese, iron, and cobalt, and/or a combination thereof, or titanium dioxide or elemental noble metals, are added to the glass melt. The glass platelets, which have a thickness of less than 1 μm and a particle size of 1 to 150 μm, can be used in blends with effect pigments in paints, varnishes, powder coatings, printing inks, plastics, and dry products, in cosmetic formulations and in decorative and care cosmetology, and also in lipophilic, hydrophilic or hydrophobic formulations. The colorants here are added to the original mixture, which is subsequently melted to produce the colored glass. This known glass coloration procedure has the major disadvantage that the metal introduced into the glass melt is soluble therein only to a level of around 0.1% by weight. The quantity figures reported in EP 1 837 379 A2 of 0.1-50% by weight of colorant in the glass platelets which are said to be obtained from a correspondingly colored glass melt are not correct. Through addition of metal compounds to the glass melt of glasses having an SiO2 content of greater than 50%, only glasses with a maximum metal concentration of 0.1% can be produced. With a higher metal concentration in the glass melt, there are instances of metal deposition in the glass melt. Owing to the low thickness of the glass pigments, a concentration of 0.1% by weight of colorant is not sufficient to produce intensely colored glass pigments or intense colors. This known method is therefore unsuitable for the production of intensely colored glass.
Disadvantageously, to date, it has not been possible to produce intensely colored glass particles, in which, however, there is great economic interest and for which there is a hitherto unsatisfied demand.